1. Field of the Invention
The invention concerns a Costas loop carrier recovery device.
2. Description of the Prior Art
A device of this kind is used in aerospace applications in a spread spectrum S band transponder for satellite orbital injection and station-keeping.
A NASA document STDN No. 203.8 entitled "Performance and design requirements specification for the second generation TDRSS user transponder" of April 1986 defines the use of a transponder of this kind whose carrier acquisition time must be less than five seconds with a probability of 90%.
The role of a Costas loop in a spread spectrum receiver is to enable coherent data demodulation and, in the case of a long loop, to offer the possibility of the transmitter being coherent with the receiver.
A loop of this kind must include a lock-on indicator and enable automatic gain control in the receiver.
The overall function to be implemented depends on constraints related to the specifications of the receiver input signal:
input frequency (slope of the variation and maximum difference); PA1 minimum signal to noise density ratio; PA1 bit timing rate; PA1 demodulation losses; PA1 loop acquisition time. PA1 a direct connection, and PA1 a parallel circuit including an amplifier in series with a filter. PA1 implementation of a frequency discriminator function so that the signal is acquired rapidly, even with a narrow loop bandwidth; PA1 implementation of a static phase error divider in the loop bandwidth to track Doppler ramps without compensation with a narrow loop and to aid fast acquisition.
These constraints are mutually contradictory and until now have obliged manufacturers to define ranges of operation dependent on the input signal to noise density ratio.
An object of the invention is to solve these problems.